The present invention relates to a display apparatus for a television receiver having a capability for simultaneously displaying two different images combined on a single display screen, with one of the images being formed in reduced size within a portion of the other image.
Television receivers are increasingly coming into use which have a capability for simultaneously displaying two images, e.g. pictures derived from two different television broadcast channels, combined upon a single display with one of the images is formed in reduced size within the other image. With such a receiver it is necessary to match the vertical scanning signals of video signals from which the two images are derived. In addition, it is necessary to incorporate an image memory, i.e. a memory which can store one frame (or one field) of video data. In recent years it has become practicable to manufacture such television receivers, due to the rapid reductions which have been achieved in the costs of digital memory devices.
FIGS. 1(a) and 1(b) illustrate two possible display arrangements for such a dual-image television receiver. In the example of FIG. 1(a), a single secondary image 12 is combined with a main image 10, with the secondary image 12 being reduced to 1/3 of the size of the main image 10 and formed within a portion of main image 10.
FIG. 2 is a general block diagram of a prior art example of such a dual-image television receiver, while FIG. 3 is a waveform diagram for illustrating the operation of the television receiver of FIG. 2. In FIG. 2, numeral 18 denotes an input terminal to which is applied a first composite video signal, 20 denotes an input terminal to which is applied a second composite video signal, 22 denotes a first video signal processing circuit, 24 denotes a second video signal processing circuit, 26 denotes a line memory (i.e. a memory device capable of storing data representing one line of video data, produced between successive horizontal sync pulses), 28 denotes an image memory, and 34 denotes a signal combining circuit for combining the video data from the first and second composite video signals to produce the main image and secondary image. This function is executed by appropriate switching operations to selectively connect a first video signal (indicated by numeral 30) and a second video signal (indicated by numeral 32) to a CRT 36.
FIGS. 3(a) and 3(e) respectively show the horizontal sync signal waveforms of the first and second composite video signals applied to input terminals 18 and 20 respectively. It is assumed in this example that the ratio of the sizes of the main image 10 and the secondary image 12 is 3:1. In order to accomplish compression of the secondary image 12 in the vertical direction, only one every three horizontal scanning lines of the second video signal is supplied as data to the line memory 26, as illustrated by waveform 3(b). In general, one in ever m lines (where m is an integer) will be selected, in accordance with the degree of size reduction applied to the secondary image. This data is transferred to the image memory 28 at the timings shown as waveform 3(c). As shown by FIG. 3(d), the lines of second video signal data are successively read out of the image memory 28 during respective time intervals which are made sufficiently short to attain the desired 3:1 compression of that data along the time axis. In this way, a 3:1 compression of the secondary image is attained along the horizontal axis of the display.
If the secondary image 302 is to be displayed at the extreme right side of the main image 10, then the data for secondary image 302 must be read out of the image memory 28 at a speed which is three times that at which the data is written into image memory 28, i.e. as illustrated by waveform 3(d). This data read-out is performed with a fixed phase relationship to the horizontal sync pulses of the first video signal. Due to the fact that read-out is performed at a higher speed in this way, it is not possible to perform write-in of data to the image memory during each read-out interval from that memory. For this reason, the transfer of data to the image memory 28 from the line memory 26 is executed during inervals in which data is not being written into line memory 26, i.e. as illustrated by waveform 3(c), and while data is not being read out of the image memory 28.
In the signal combining circuit 34, second video signal 32 is selected to be applied to the CRT during intervals in which the signal shown in FIG. 3(d) is at the high logic level and the first video signal 30 is selected during intervals in which the waveform of FIG. 3(d) is at the low logic level. In ths way, the main image 10 and secondary image 12 are combined to form a dual-image display on the screen of CRT 36 as shown in FIG. 1(a).
However with a prior art television receiver having the configuration described above, the problem arises of limitations on positions (along the horizontal axis of the display) at which the secondary image 12 can be displayed. Comparing waveforms 3(c) to 3(e), then when a secondary image is to be displayed which is smaller than the main image by a factor of 3:1, if each of the time intervals in which the data for successive lines of the secondary image is not made equal to or less than 1/4 of the horizontal scanning interval of the main image, so that the maximum data sampling time is 3/4 of that horizontal scanning interval, then the requirements described above for producing the secondary image within the main image will not be satisfied. As a result, there will be insufficient data transferred to the CRT, i.e. that all of the data for each line of the secondary image will not be transferred to the CRT during each CRT scanning line, and portions at the left or right side of the secondary image may not appear on the display. Furthermore, it would be impossible in principle with such a prior art television receiver to produce a multi-image display of the form shown in FIG. 1(b), in which a plurality of secondary images, e.g. secondary images 13 and 16, are displayed in combination with a main image 14. The contents of each of these secondary images is identical.